Postgres 11

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Help with an interesting Postgres question: Why isn't an Index Only Scan used on a partition accessed via the parent table?.

create table objects (obj_id int, obj_type character varying);

insert into objects values
(1, 'val 1'),(2, null),(3, 'val 3'),(4, 'val 4');

CREATE TABLE

INSERT 0 4

SELECT '  (' || obj_id || ', ''' || coalesce(obj_type, '') || '''),' 
FROM   objects 

?column?

  (1, 'val 1'),

  (2, ''),

  (3, 'val 3'),

  (4, 'val 4'),

SELECT 4

SELECT format('(%s, %L),', obj_id, obj_type)
FROM   objects;

format

(1, 'val 1'),

(2, NULL),

(3, 'val 3'),

(4, 'val 4'),

SELECT 4

SELECT o  -- whole row
FROM   objects o
WHERE  obj_id < 11
ORDER  BY obj_id;

o

(1,"val 1")

(2,)

(3,"val 3")

(4,"val 4")

SELECT 4